Submersible pumping systems are often deployed into wells to recover petroleum fluids from subterranean reservoirs. Typically, a submersible pumping system includes a number of components, including an electric motor coupled to one or more high performance pump assemblies. Production tubing is connected to the pump assemblies to deliver the petroleum fluids from the subterranean reservoir to a storage facility on the surface. The pump assemblies often employ axially and centrifugally oriented multi-stage turbomachines.
Most downhole turbomachines include one or more impeller and diffuser combinations, commonly referred to as “stages.” The impellers rotate within adjacent stationary diffusers. A shaft keyed only to the impellers transfers mechanical energy from the motor. During use, the rotating impeller imparts kinetic energy to the fluid. A portion of the kinetic energy is converted to pressure as the fluid passes through the downstream diffuser.
Although widely used, conventional downhole turbomachinery is vulnerable to “gas locking,” which occurs in locations where petroleum fluids include a significant gas to liquid ratio. Gas locking often causes the inefficient operation or complete failure of downhole turbomachinery. The gas-locking phenomenon can be explained by the dynamics of fluid flow through the impeller and diffuser. As gas and liquid pass through the channels of a diffuser, its flow directions are guided by curved vanes. The change of flow directions usually generates relatively high and low pressure zones in the flow channels. The streamwise and transverse pressure gradients, streamline curvature and slip between different phases contribute to the segregation of the phases. Gas bubbles tend to move into low pressure zones because of the hydrodynamic behavior of bubbles in liquids. When the two-phase mixtures exit the diffuser, there tend to be more bubbles in the low pressure zones than in the high pressures zones. In severe cases, phase separation can occur in the flow. Upon separation, the gas phase tends to accumulate in certain regions of the flow passage, causing head degradation and gas locking.
In particular, fluid exiting the diffuser and entering the impeller eye often experiences a pressure drop that is usually higher on the shroud side of the vane at the time of entrance to the vanes of the impeller. This pressure drop increases the separation of gas components from liquid components within the fluid. Centrifugal force tends to carry the heavier liquid components to the outer regions of the impeller while the lighter portions concentrate toward the interior of the impeller eye. Gas locking typically begins at the inlet suction side of the vane and extends the accumulation of the increased bubble size to the hub end of the impeller to complete the gas locking of the pumping system.
There is therefore a continued need for an improved pump assembly that effectively and efficiently produces two-phase fluids from subterranean reservoirs. It is to these and other deficiencies in the prior art that the present invention is directed.